High Ohms Measurements; 4-Wire Ohms - Keithley 2790 User Manual

Model 2790 SourceMeter

High ohms measurements

The 7751 and 7753 include a programmable V-source (50 to 500V), so the constant volt-
age method can be used to measure high resistance (such as insulation resistance). The
Model 2790 mainframe does not have a voltage source.
If not using a 7751 or 7753 module in your test system, you can use the Ω2 function
(100MΩ range) of the Model 2790 to measure insulation resistance up to 120MΩ.

4-wire ohms

4-wire ohms eliminates the effects of test lead resistance when measuring a low ohms
DUT.
ohms DUT.
If the test lead resistance (R
DUT (R
test leads. Therefore, the measured voltage (which is used to calculate resistance) will not
be the same as the actual voltage (V
Figure 6-1
Using Ω Ω Ω Ω 2 function to measure resistance
Keithley 2790
I
SOURCE
(1mA)
Ω2 Function Selected
®
Switch System User's Manual
Figure 6-1 shows how 2-wire ohms can be adversely affected when measuring a low
LEAD
), the measured voltage (V
DUT
Input HI
v
V
MEAS
) is relatively large in comparison to the resistance of the
) will include the voltage drops across the two
MEAS
) across RDUT.
R
R
LEAD
V
MEAS
R
LEAD
Input LO
V = Voltage measured by 2790 DMM
MEAS
V = Voltage across RDUT
R
V
MEAS
R = = R
MEAS DUT
I
SOUR
V
R
R =
DUT
I
SOUR
Test Current
V R
R DUT
+ (2 x R )
LEAD
6-3